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Cardboard tube working as variable resonator


Andres Ramos

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A nice expewriment from today was an idea coming from discussions with Jeff about applying cardboard tubes as bandpass resonators for human speech. It was a very simple setup. I took a piece of cardboard tube, put in a buzzer into one end (working on a steady frequency), covered this end with one hand and with the other hand I opened and closed the upper end of the tube in a random manner.

Basically the results are promising ic combined with random excitation of the buzzer.

 

 

 

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4 hours ago, Christine Vogts said:

Well..if I understand Chicken cacophony...

It sounds like in the very beginning... "Come back and play dominoes Ramos"..

🙂

Really? Sometimes my wife and me are playing 'Triomino', a variant of Domino with triangular shaped stones. We want to play it again at Christmas Eve this year.

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Andres, dare we say you invented another hand instrument? It might be monotone but there's potential 🙂

perhaps a way to modulate without a hand would be to setup a fan blowing towards the tube? How could we randomize something like this? 

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6 hours ago, Keith J. Clark said:

Andres, dare we say you invented another hand instrument? It might be monotone but there's potential 🙂

perhaps a way to modulate without a hand would be to setup a fan blowing towards the tube? How could we randomize something like this? 

Hm yes, you're playing with the idea of a mechanical cavity resonator that can tuned quickly by a randomized signal source. A tricky challenge that requires a bit of mechanical work. Basically I see the following options.

1. Changing the resonators volume

Let's say we take a piece of a cardboard tube like from a toilet paper roll. We put a loudspeaker or some kind of other mechanical vibrator at one end that is sealed by a diaphragm. In the cardboard tube Wall we cut a half inch hole to let out the sound. The second end of the tube gets closed by a moving piston that is driven by some motor or so. The piston should be made from styrofoam to minimize it's mass in order to move it quickly. For first tests we could use the motor with something like a crankshaft to move the piston back and forth. The result would be a quick sine sweep of the resonance frequency. If this works we must make up our minds to control the piston freely.

2. Making a flute like resonator

Again we could take a cardboard tube with diaphragm and vibrator/loudspeaker at one end. Second end is closed by a lid with a small hole in it to let out the sound. Also we cut a hole in the wall but this time it is closed by a piece of rubber attached to a motor shaft or solenoid. These can be used to more or less close/uncover the hole. This will change the pitch of the resonance frequency like with a flute.

For the vibrator I would recommend a standard buzzer that generated a signal close to glottal impulses.

 

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3. A combination of 1 and 2.

Do you remember those throttle valves in the old combustion engines having a carburetor? It was a disk with the size of the inner diameter of the air intake tube. The disk was fixed to a shaft going through the tube walls. By turning the shaft by an angle between 0 and 90° you could change the tube opening between fully closed to fully opened. In our case this would sweep the resonance frequency as well. The shaft could be directly driven by a motor and the motor modulated by a power transistor. In my eyes this is the most promising approach.

 

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